The field of the present invention is drive mechanisms for power boats. More specifically, the present invention is directed to mechanisms for transmitting power from an inboard motor of a power boat to a propeller.
A variety of mechanisms for providing power to a propeller for driving a boat have been employed, both successfully and unsuccessfully, since at least the 1800's. Two general categories of such devices employed with inboard motors have developed. Early on, fixed propeller shafts were developed which generally require a second mechanism, a rudder, for steering. More recently, devices known as inboard-outboards or stern drives have been developed which employ an articulated propeller shaft coupled with an inboard motor. These devices do not generally require additional steering mechanisms as the thrust from the propeller or propellers may be directed to effect steering much as a conventional outboard motor is employed.
Two mechanisms may be considered representative of the types of stern drives presently available. The first is illustrated in the North patent, U.S. Pat. No. 3,136,287. This device employs a horizontal input shaft, a vertical power transmission shaft, and a horizontal propeller shaft. This type of stern drive provides certain advantages of outboard motor flexibility and steering control. However, vertical shaft stern drives are typically rather inefficient because of the required gearing. The second type is represented by the Adams et al. patent, U.S. Pat. No. 3,933,116. This patent employs an inclined shaft from the inboard motor to an outboard, articulated propeller shaft. For reasons pointed out below, the engine disadvantageously must be placed low in the bilge, must be inclined and must be forward in the boat, particularly if gearing is required.
Coincident with the development of power trains for boats, improved propeller performance has also been achieved. To date, it is understood to be beneficial to run the propeller of high speed, competition type boats only about 55% submerged in the water. It is also believed to be beneficial to have the axis of the propeller angled downwardly relative to the keel line by a maximum of about two to three degrees. Finally, a factor affecting the overall performance of the propeller and the hull is to have a relatively low center of thrust. Because of these desirable factors for high speed performance, the inboard motor of the Adams et al. device must be located low, at an angle, and as far forward as possible. The North type stern drive is better able to accomplish a desired propeller orientation. However, as mentioned above, substantial efficiency is lost in the power train.